The field of exoplanets has made tremendous advances since the first discovery of a planet orbiting a distant sun-like star in 1995. Thousands of exoplanets and planet candidates have been detected to date, and observational techniques have improved sufficiently to allow for the detection planets smaller than Neptune (4 Earth-radii, 17 Earth-masses). Although we cannot (yet) send a rover or satellite to observe these distant planets directly, there is still a great deal of information we can gather about these bodies. The mass of a planet can be estimated from the radial velocity wobble its gravitational pull induces in its host star. The radius can also be measured if the planet transits, crossing in front of the disk of its star and decreasing the apparent brightness of the star as observed from Earth. Planets detected through both radial velocity and transit observations are especially valuable, since with both mass and radius measurements we can gauge the planet's average density and learn about its interior composition.

Over the past seven years, transiting sub-Neptune-size planets have gone from a purely theoretical construct, to a handful of discoveries, to a planet category with more instances than can be remembered by name. One of the most striking revelations from the Kepler exoplanet transit survey was the plethora of small planets discovered; 80% of the 3845 transiting planet candidates identified by Kepler have radii smaller than Neptune (Borucki et al. 2011, Batalha et al. 2012). Though they are abundant around distant stars, planets with radii intermediate between Neptune and Earth have no analogs within our Solar System.

The nature of planets in this regime is not yet known; terrestrial super-Earths, mini-Neptunes with hydrogen gas layers, and ocean-worlds with vast quantities of water are all plausible compositions. Disentangling the contributions from each of these scenarios to the population of observed planets is a critical missing link in our understanding of planet formation, evolution, and interior structure.

One of the main objectives of my research is to interpret the measured properties of sub-Neptune exoplanets, employing models to constrain the planets' bulk compositions, formation histories, and habitability. I have developed a computer model for the internal structure of low-mass exoplanets (Rogers & Seager 2010a), produced the first bulk composition constraints for the transiting super-Earth GJ1214b (Rogers & Seager 2010b), explored core-nucleated accretion and rocky planet outgassing as potential formation pathways for the Neptune-size planet candidates found in abundance by Kepler (Rogers et al. 2011), constrained the scenarios in which sub-Neptune planets with voluminous gas layers may harbor surface liquid water oceans, and placed statistical constraints on the fraction of planets that are rocky as a function of planet size (Rogers 2014). My models of the transiting super-Earth GJ1214b (Rogers & Seager 2010b) helped to motivate follow-up observations with the Hubble Space Telescope (Berta et al. 2011), the Spitzer Space Telescope (Desert et al. 2011), and ground based observatories (e.g., Bean et al. 2011, Croll et al. 2011).

While there has been substantial progress, this newly discovered planet category is ripe with outstanding puzzles and scientific opportunities!

Planet Microlensing

Microlensing is an approach to discover exoplanets that relies upon chance alignments between two stars along the line of sight to Earth. The gravitational field of the foreground star acts as a lens, magnifying the light from the background star. Anomalies in the magnification light curve can reveal planets orbiting the lens star. I have collaborated with Paul Schechter to calculate cross-sections for various categories of planetary microlensing events. Hereis a pedagogical microlensing poster I made for the IAU 276 Symposium.

Interacting Binary Stars

I became interested in transiting white dwarfs after Kepler discovered two very unusual transiting objects in its first 43 days of science photometry. These curious transiting companions, named KOI-81 and KOI-74, are planet-sized (with radii similar to Jupiter), while also being hotter than their stellar hosts (with effective temperatures in excess of 10,000K). KOI-81 and KOI-74 are likely white dwarfs – the degenerate cores of dead stars that have lost their outer layers. With Saul Rappaport, I am performing a stellar population synthesis calculation to compute occurrence rates for white dwarfs transiting main sequence stars, and to predict the statistical distributions of masses, periods, and transit depths for these binary star systems. Comparing the outcomes of this calculation to the eventual full Kepler transit sample will help to confirm whether KOI-81 and KOI-74 are indeed white dwarfs, and may lead to new insights into the physics governing interacting stellar binaries.

Meteoroid Ablation

Observations of the 1998 Leonid meteor storm yielded exceptional meteors that have helped to push the boundaries of meteoroid ablation theory. While conventional meteor ablation altitudes range between 80−125 km, meteors having high beginning heights up to 200 km were detected. Whereas other observational studies have found meteors with transverse spread on the order of meters, very wide meteors exhibiting comet-like diffuse structures on the order of a kilometer were detected among the 1998 Leonids. Meteoroid ablation models have typically assumed that the destruction of a meteoroid in the Earth’s atmosphere is primarily a thermally driven process: the meteoroid evaporates following intense heating during atmospheric flight. Light is emitted when excited atomic and molecular states produced by collisions between ablated meteor atoms and atmospheric constituents decay. However, thermal ablation cannot account for the exceptional meteors observed during the 1998 Leonid meteor storm.